extract duplicate logic into a variable on _commit_message_container

app/views/shared/_commit_message_container.html.haml

 .form-group.commit_message-group
   - nonce = SecureRandom.hex
+  - descriptions = local_assigns.slice(:message_with_description, :message_without_description)
   = label_tag "commit_message-#{nonce}", class: 'control-label' do
     Commit message
   .col-sm-10
@@ -8,14 +9,14 @@
       = text_area_tag 'commit_message',
           (params[:commit_message] || local_assigns[:text] || local_assigns[:placeholder]),
           class: 'form-control js-commit-message', placeholder: local_assigns[:placeholder],
-          data: local_assigns.slice(:message_with_description, :message_without_description),
+          data: descriptions,
           required: true, rows: (local_assigns[:rows] || 3),
           id: "commit_message-#{nonce}"
     - if local_assigns[:hint]
       %p.hint
         Try to keep the first line under 52 characters
         and the others under 72.
-    -if local_assigns.slice(:message_with_description, :message_without_description).present?
+    - if descriptions.present?
       %p.hint.with-description-hint
         = link_to "#", class: "with-description-link" do
           Include description in commit message